There is provided a method for preparing an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal. This modified solid oxide may be used as a catalyst, for example, to isomerize C.sub.4 to C.sub.8 paraffins. The modified solid oxide is prepared by co-precipitating the Group IVB metal oxide along with the oxyanion of the Group VIB metal.
The present catalysts, especially tungstate-modified zirconia catalysts, are useful for many hydrocarbon conversion processes such as paraffin isomerization, ring opening of cyclics, alkylation of naphthalene, hydrocracking, etc. These catalysts may be prepared by a complex impregnation procedure involving numerous steps including the precipitation of zirconia precursor, refluxing, two filtration/washing/reslurry steps, impregnation with tungsten, and a final calcination. In accordance with the present method, this complex procedure is reduced to just three steps: co-precipitation of tungsten with the zirconia precursor, filtration, and calcination.
The present method produces a better catalyst at lower manufacturing cost (fewer steps) and is more flexible to address the environmental concerns (eliminates the requirement for a chloride salt).
The solid material described herein comprises an oxide of a Group IVB metal, preferably zirconia or titania. This Group IVB metal oxide is modified with an oxyanion of a Group VIB metal, such as an oxyanion of tungsten, such as tungstate. The modification of the Group IVB metal oxide with the oxyanion of the Group VIB metal imparts acid functionality to the material. The modification of a Group IVB metal oxide, particularly, zirconia, with a Group VIB metal oxyanion, particularly tungstate, is described in U.S. Pat. No. 5,113,034; in Japanese Kokai Patent Application No. Hei 1 1989!-288339; and in an article by K. Arata and M. Hino in Proceedings 9th International congress on Catalysis, volume 4, pages 1727-1735 (1988), the entire disclosures of these publications are expressly incorporated herein by reference. According to these publications, tungstate is impregnated onto a preformed solid zirconia material.
For the purposes of the present disclosure, the expression, Group IVB metal oxide modified with an oxyanion of a Group VIB metal, is intended to connote a material comprising, by elemental analysis, a Group IVB metal, a Group VIB metal and oxygen, with more acidity than a simple mixture of separately formed Group IVB metal oxide mixed with a separately formed Group VIB metal oxide or oxyanion. The present Group IVB metal, e.g., zirconium, oxide modified with an oxyanion of a Group VIB metal, e.g., tungsten, is believed to result from an actual chemical interaction between a source of a Group IVB metal oxide and a source of a Group VIB metal oxide or oxyanion.
This chemical interaction is discussed in the aforementioned article by K. Arata and M. Hino in Proceedings 9th International Congress on catalysis, volume 4, pages 1727-1735 (1988). In this article, it is suggested that solid superacids are formed when sulfates are reacted with hydroxides or oxides of certain metals, e.g., Zr. These superacids are said to have the structure of a bidentate sulfate ion coordinated to the metal, e.g., Zr. In this article, it is further suggested that a superacid can also be formed when tungstates are reacted with hydroxides or oxides of Zr. The resulting tungstate modified zirconia materials are theorized to have an analogous structure to the aforementioned superacids comprising sulfate and zirconium, wherein tungsten atoms replace sulfur atoms in the bidentate structure.
Although it is believed that the present catalysts may comprise the bidentate structure suggested in the aforementioned article by Arata and Hino, the particular structure of the catalytically active site in the present Group IVB metal oxide modified with an oxyanion of a Group VIB metal has not yet been confirmed, and it is not intended that this catalyst component should be limited to any particular structure.